There are unused 3 AND, 1 NOT and another 1 NOT + LED may be recovered at expense of HALT visualisation.

Also under SD card module is 8x9 free THT soldering points accompanied by +5V, +3V3 and 2 GND.

All MCU pins are avaiable on connectors as well as lot of internal points.

That can be used for adding custom connections, HW and functionality, if needed.